Steel materials for medium-sized or large-sized bearing parts include bearing steels typified by SUJ3 and SUJ5 specified in JIS G 4805 (2008), and SNCM-based case hardening steels typified by SNCM815 specified in JIS G 4053 (2008). One example of the production process of bearing parts utilizing such steels is as follows. A steel material is subjected to hot working (for example, hot forging), and cutting to produce an intermediate product having a desired shape. The intermediate product is subjected to heat treatment so as to have predetermined hardness and microstructure. The heat treatment is quenching and tempering in the case of a bearing steel, and carburizing treatment (carburizing-quenching and tempering) in the case of a case hardening steel. A bearing part is produced by the processes described above.
For some bearing parts, excellent wear resistance and surface-originated flaking life are required. In such a case, as the above described heat treatment in the production process of bearing parts, carbonitriding treatment (carbonitriding-quenching and tempering) is performed in place of carburizing treatment. Carbonitriding treatment increases carbon and nitrogen concentrations in an outer layer of the steel material, thereby hardening the outer layer of the steel material.
Techniques relating to bearing parts or steel materials for bearing are proposed in Japanese Patent Application Publication Nos. 08-49057 (Patent Literature 1), 2008-280583 (Patent Literature 2), 11-12684 (Patent Literature 3), and 06-287712 (Patent Literature 4).
In Patent Literature 1, carburizing treatment or carbonitriding treatment is performed on a steel material containing a large amount of V (vanadium) to cause V carbide to precipitate in an outer layer. It discloses that this V carbide provides excellent wear resistance for a rolling bearing.
However, the V content in the steel material of Patent Literature 1 is as high as 0.8 to 2.0%. For that reason, performing carbonitriding treatment may cause coarse V carbides and V carbonitrides to be formed, thereby significantly deteriorating toughness of the bearing member.
In the case hardening steel for bearing disclosed in Patent Literature 2, paying attention to hydrogen embrittlement effect on bearing life, V carbides are finely dispersed, thereby increasing the effect of hydrogen trap site. It discloses that this increases surface fatigue strength.
However, in the technique proposed in Patent Literature 2, toughness may deteriorate along with the amounts of S and P contained in the steel material.
In the case hardening steel for cold forging disclosed in Patent Literature 3, for the purpose of cost reduction, the chemical components and microstructure of steel are adjusted to accelerate spheroidizing annealing treatment.
However, when producing a bearing by using the steel material proposed in Patent Literature 3, hardenability and toughness may be insufficient.
A steel part disclosed in Patent Literature 4 contains a large amount of retained austenite in an outer layer as a result of performing carbonitriding quenching. It is stated that as a result of this, a surface-originated flaking life is improved.
However, the steel material of Patent Literature 4 contains not less than 0.5% of expensive Ni. For that reason, its production cost increases when applied to medium-sized and large-sized bearing parts.
In recent years, there is growing demand for longer lives of bearing parts under impact environments, poor lubrication environments, and high interfacial pressure conditions, and it is desired to establish enabling technology which excels in toughness, wear resistance, and surface-originated flaking life. A conventional practice to increase toughness and surface-originated flaking life has been to perform carburizing or carbonitriding on a case hardening steel typified by SCM and SNCM of JIS G 4053 (2008), and a case hardening steel in which alloy elements such as Si, Mn, Mo, and V are optimized, thereby increasing the retained austenite. However, since the retained austenite is a soft structure, increase in the amount of retained austenite will deteriorate wear resistance.